1. Field of the Invention
The present invention relates to a system for a multiple-conversion of digital data having one frequency to digital data having another frequency. More particularly, it relates to a system for a multiple-conversion of digital data having a frequency of, for example, 1.544 MHz, at a first order group in a digital data communication system, to digital data having another frequency of, for example, 6.312 MHz, at a second order group in the digital data communication by using a digital stuffing method.
The digital data multiple conversion system of the present invention is preferably used in terminal equipment in the digital data communication system.
2. Description of the Related Art
Recent advanced digital data communication networks can be constructed in a hierarchy structure, and such a digital data communication network comprises first to fifth order groups. Note, occasionally, a digital data communication network will comprise a one and a half (1.5) order group. In short distance data communication, data issued from a data processing equipment is multiplexed at an exchange office, fed to a transmission line of the first order group through the exchange office, further multiplexed at a terminal equipment, and then fed to a transmission line of the second order group. When the data approaches a target data processing equipment, the data on the transmission line of the second order group is demultiplexed at another terminal equipment, fed to a transmission line of the first order group different to the above first order group transmission line, demultiplexed at another exchange office, and finally transferred to the target data processing equipment. If longer distance data communications are required, transmission lines of higher order groups, for example, the third order group and the fourth order group, can be used for the data transfer.
The data transfer speeds are as follows: 1.544 Mega-bit-per-seconds (Mbps) for the first order group; 3.152 Mbps for the 1.5 order group; 6.312 Mbps for the second order group; approximately 32 Mbps for the third order group; approximately 98 Mbps for the fourth order group; and, approximately 400 Mbps for the fifth order group. The relationship of the data transfer speed between the first and 1.5 order groups is approximately 1:2, approximately 1:4 between the first and second order groups, approximately 1:2 between the 1.5 and second order groups, and so on. Therefore, for example, the data on the transmission line of the first order group is multiplexed at a 1:4 relationship at the terminal provided between the first and second order groups, and fed to the transmission line of the second order group, and transferred at 6.312 Mbps.
However, the relationship of the data transfer speed is not a fine integer. For example, a four time multiple of the first order group's transfer speed of 1.544 Mbps is 6.176 Mbps, but the transfer speed of the second order group is 6.312 Mbps. This irregular relationship of the data transfer speed lying between two groups is based upon a difference of the provision of redundant data in each group, which must be inserted for data transfer and is defined by a data format of each group.
Upon multiplexing or demultiplexing, the problem of the above irregular relationship of the data transfer speed, i.e., an irregular relationship of a frequency between two groups, may be solved by providing a phase-locked-loop (PLL) circuit, because a PLL circuit can continuously change the frequency. However, the provision of the PLL circuit necessitates a complicated circuit structure and high costs.
Also, stuffing, which bypasses data multiplexing or data demultiplexing under certain conditions, can be applied to adjust the above irregular frequency relationship. However, a prior art multiplexing or demultiplexing still suffers from a disadvantage of an irregular multiplexing or demultiplexing due to an omission of pulses. This will be described later with reference to the drawings.